In case of the ranging by this type of distance measuring apparatus, for example, a stereo camera is generally expensive because it is necessary to match optical features of left and right cameras. On the other hand, if a monocular camera is used to reduce the cost, although the ranging is possible from the principle of triangulation, it is necessary to obtain the displacement parameter of the imaging element: the three-axis amount of rotation and the three-dimensional amount of parallel displacement. In specifying the displacement parameter, there is known a technology in which several feature points are selected from taken images and in which the displacement of the feature points is focused on to specify the displacement parameter. If noise is selected as the feature point, it is hardly possible to ignore an influence of errors. Thus, there is a possibility that the displacement parameter cannot be accurately obtained.
Thus, there is suggested a technology of searching for the displacement of the feature point between the images, evaluating the errors, and statistically detecting the displacement parameter (refer to a non-patent document 1).
There is also suggested a technology of using the displacement of the vanishing point of horizontal lines which are parallel to each other and of the vanishing point of perpendicular or vertical line segments which are parallel to each other, in order to separately obtain the amount of rotation and the amount of parallel displacement, of the displacement parameter (refer to a non-patent document 2).
Moreover, there is also suggested a technology of detecting the vanishing point by using a white line on a road (refer to a patent document 1).    Non-patent document 1: IEEE TRANS. ON SYSTEM, MAN AND CYBERNETICS, VOL. 19, NO. 6, NOVEMBER/DECEMBER 1989 pp. 1426-1446    Non-patent document 2: Journal of Institute of Electronics and Communication Engineers of Japan, '86/6 VOL. J-69-D NO. 6 pp. 967-974    Patent document 1: Japanese Patent Application Laid Open No. Hei 7-78240